Double-sided magneto-optical recording disk

ABSTRACT

A magneto-optical disc is described. The magneto-optical disc comprises a substrate and a magneto-optical recording layer applied over the substrate. The magneto-optical disc further comprises an overcoat layer applied over the recording layer having a thickness in a range between 25 nanometers and 200 microns, wherein data is recorded on the magneto-optical disc and read from the magneto-optical disc using a laser light that is incident on the overcoat layer.

The present application is related to application Ser. No. 09/027,420 filed Feb. 20, 1998, and Ser. No. 09/026,798 filed Feb. 20, 1998, both of which are assigned to the assignee of the present application.

FIELD OF THE INVENTION

The present invention is in the field of magneto-optical data storage discs. The present invention is more particularly in the field of magneto-optical storage discs for use with an overcoat incident recording technique.

BACKGROUND OF THE INVENTION

Typical magneto-optical (MO) disc drives record data by locally heating a portion of the disc. MO discs, or media, include a recording layer of a magnetic material. The coercivity of the heated portion of the media is lowered when it is heated by the laser beam. This allows the magnetic polarity in that area to be reversed by an applied magnetic field. In such disc drives, data is read from media by illuminating areas of the storage media with a linearly polarized laser beam. The Kerr rotation effect causes the plane of polarization of the illuminating beam to be rotated. The direction of rotation depends on the magnetic polarity in the illuminated area of the storage media. When the disc is read, the polarization rotation is determined with a pair of optical detectors and a polarization beam splitter to produce an output data signal. Limitations of MO disc drives include data access time and density with which data can be stored.

FIG. 1 is a diagram of prior MO recording system 100 typically used with 130 millimeter (mm) diameter MO media. System 100 is an example of a “substrate incident” recording system. In substrate incident systems, laser light is incident on a thick substrate, travels through the substrate, and is focused on a recording layer below the substrate. System 100 includes objective lens 102 for focusing a collimated beam of light on disc 116. Disc 116 is an example of a typical two-sided MO disc. MO disc 116 includes substrates 104 and 114 forming outside layers on opposing sides of disc 116. Substrates of 104 and 114 are made of materials such as plastic polycarbonate and are approximately 1.2 mm thick. Recording layer 106 is below substrate 104, and recording layer 112 is below substrate 114. Recording layers 106 and 112 can be made out of any one of a number of well-known materials, such as Tb—Fe—Co, a rare-earth transition-metal alloy. The laser light beam passing through objective lens 102 penetrates substrate 104 as shown and is incident on a focal point on the surface of recording layer 106.

System 100 has several disadvantages. One of the disadvantages of system 100 is that it is necessary to apply energy to the recording layer to erase data prior to writing new data. This is because a large, stationary magnetic coil (not shown) having a large inductance is situated on the opposite side of disc 116 from objective lens 102 to assist in the writing process. Because the coil is held at a relatively great distance form the media surface and has a relatively large inductance, the magnetic field cannot be reversed at high frequencies. Therefore, it is necessary to erase data before writing new data. The necessity of erasing before rewriting slows the process of writing data to disc 116.

Another disadvantage of system 100 is that the density of data stored on disc 116 is relatively low. A further disadvantage of system 100 is that only one side of disc 116 can be accessed at one time. This is because the relatively large coil occupies the space on the side of the disc opposite the objective lens. This space cannot therefore be used for another lens and actuator. In order to access a different side of disc 116, disc 116 must be removed, turned over, and reinserted into system 100. Disc 116, however, provides good data security because relatively thick substrates 104 and 114 allow disc 116 to be handled without danger of data loss or difficulty in reading data because of contamination. The thickness of substrates 104 and 114 presents difficulties, however. Because the substrate material is applied so thickly, non-uniform heating and cooling characteristics at the outer radii cause non-uniform solidification during the molding process. Therefore, light passing through the substrate at the outer and inner radii is distorted such that birefringence occurs. The edges of the disc are thus unsuitable for data storage, and the data storage capacity of the disc is reduced. In addition, typically, with a thick substrate, some degree of light distortion is experienced throughout the disc.

Another disadvantage of system 100 is its susceptibility to tilt. Tilt is the deviation from 90 degrees of the angle between the axis of incident light and the plane of the recording layer of the media. Because incident light must pass through a relatively thick substrate before reaching a recording layer, the incident light becomes distorted with the occurrence of tilt. This effectively limits the possibility of reducing the spot size of the incident light, and consequentially effectively limits the possibility of increasing the storage density of the MO disc. It is desirable to decrease the spot size because a smaller spot size corresponds to a larger numerical aperture (NA) of the system. There are factors that place an upper limit on NA. One of the factors is tilt. Therefore, in a system such as system 100, susceptibility to tilt limits the increase of NA and limits storage capacity of the disc.

FIG. 2 is a diagram of another prior MO recording system 200. Collimated light beam 202 passes through objective lens 204 to disc 216. Disc 216 includes substrate 206 that is typically 0.6-1.2 mm thick. Disc 216 further includes recording layer 208 between substrate 206 and protective layer 210. In system 200, the large, stationary coil of system 100 is replaced by a relatively small coil in flying magnetic recording head 214. Flying height 212 is maintained by an air bearing created when disc 216 passes under flying magnetic recording head 214. For writing to disc 216, a magnetic field created by magnetic recording head 214 is used in conjunction with collimated light 202 which passes through objective lens 204. The smaller coil of magnetic recording head 214 has less inductance than the large, stationary coil of system 100. The reduced inductance allows direct overwrite of data on disc 216 by switching the magnetic field.

System 200 still possesses the disadvantage of relatively low storage densities. For example, because system 200 is a substrate incident system, the thick substrate 216 causes system 200 to have the same susceptibility to tilt as system 100. In addition, disc 216 is a one-sided, rather than a two-sided disc, which reduces overall storage capacity.

System 200 also has the disadvantage of requiring mechanical coupling of light on one side of disc 216 with magnetic recording head 214 on the other side of disc 216. Typically, this coupling is accomplished by mechanical linkages that pass from objective lens 202 to magnetic recording head 214 around the edge of disc 216. The mechanical linkages cannot be allowed to interfere with the movement of objective lens 202 or with disc 216.

FIG. 3 is a diagram of prior MO recording system 300. System 300 is an example of an “air incident” design in which a lens is held very close to the media and laser light is incident on very thin protective layer 309 that is over recording layer 308 of disc 318. Other prior art air incident designs may have no protective layer, wherein a surface of recording layer is the surface of the disc. System 300 employs flying magnetic recording head 316, and a two-piece objective lens comprised of lens 314 and lens 312. Prior art systems similar to system 300 use other lens designs, for example, single lens or three-piece objective lens designs. Lens 314 is held extremely close to disc 318. Collimated light beam 302 passes through lens 312 and lens 314. Lens 312 and lens 314 are integrated with slider 304 and magnetic recording head 316. Flying height 306 for system 300 is typically less than the wavelength of the laser light used in reading from and writing to MO disc 318.

Disc 318 has an MO recording layer 308 over substrate 310. Because in system 300, flying objective lens 314 is in close proximity to disc 318, the need for a focus actuator is eliminated. As is known, focus actuators are mechanisms that adjust the height of an objective lens over a disc during read and write operations. In the case of system 300, the height of flying lenses 312 and 314, and thus the focus of flying objective lenses 312 and 314, are determined by the distance between recording layer 308 and flying objective lenses 312 and 314 as determined by the air bearing created between slider 304 and disc 318 during flight.

By maintaining the spacing between flying objective lenses 312 and 314 and recording layer 308 at less than the wavelength of the laser light used, laser light can be focused in the near field mode of operation. As is known, the near field mode of operation uses the phenomenon of evanescent coupling, which requires that the objective lens be held very close to the recording layer. The use of evanescent coupling to perform recording allows a smaller spot size, and therefore, greater recording densities and better data throughput.

System 300 has several disadvantages. For example, the surface of very thin protective layer 309 or the surface of lens 314 closest to the disc can be contaminated, causing permanent damage to data or to the disc drive system.

Another disadvantage of system 300 stems from the fact that because there is one objective lens and no focus actuator, the flying height must be tightly controlled to adequately control focus. Variations in the flying height and thickness of very thin protective layer 309 over the recording layer must be controlled within the depth of focus tolerance of the flying lens. Generally, the tolerance of flying height 306 and protective layer 309 thickness is a percentage of the nominal thickness. Therefore, in order to reduce the tolerance, the nominal thickness of very thin protective layer 309 must be reduced. For example, the depth of focus tolerance is generally plus or minus 0.5 micron. A typical tolerance in applying very thin protective layer 309 is ten percent of the thickness of the protective layer. Therefore, flying height 306 and the thickness of very thin protective layer 309 together must be very small for the thickness variation of very thin protective layer 309 to be less than 0.5 micron.

In the case of a near field system such as system 300, the flying height (the distance between the bottom surface of flying lens 314 and the surface of recording layer 308) must be less than the wavelength of the laser light. The wavelength of the laser light is typically 700 nanometers. Therefore, the thickness of a protective layer on recording layer 308 would have to be on the order of 25 nanometers. This is extremely thin and would not protect data on recording layer 308 from manual handling in a removable disc application, or from corrosion or contamination during shelf life. Even with the protection of a cartridge that covers disc 318, some contamination from particles in the atmosphere or from humidity or corrosive gases is inevitable over time.

Another disadvantage of system 300 is that this system may require lubrication of the surface of the magnetic media because the extremely low flying height creates a possibility of contact between the media and the head. A lubricant is used to minimize friction on contact and possibly to prevent “head crash” onto the media. Providing the lubricant is an expense and the application of the lubricant is an additional step in the manufacturing process. The presence of the lubricant is also a disadvantage because it does not allow the media to be wiped while in the drive mechanism for the purpose of removing contaminants from the surface.

Conventional disc drives all share similar disadvantages related to access of data on a storage disc. Current disc drives, even those designed to access two-sided media, are limited to accessing one side of the media at a time. It has not been possible, previously, to simultaneously and independently access both sides of a two-sided disc. One of the reasons for this is that reading/writing head mechanisms on either side of the disc are constructed to move together or not at all. Current disc drives therefore have limited data access speeds. This disadvantage is shared by previous MO drives and drives using other technologies, such as those used in computer hard disc drives.

Technology exists to make multiple disc drives appear to a client device as a single drive. Redundant arrays of independent drives (RAIDs) divide incoming data into multiple streams which are written to multiple drives simultaneously. RAIDs can be used to increase throughput by dividing a single incoming data stream and writing portions of it to multiple drives simultaneously. RAIDs can also be used to achieve data redundancy by sending different copies of the same data to multiple drives simultaneously. Although access speed can be increased by using RAIDs, RAIDs are expensive and complex because they are merely devices containing duplicate conventional disc drives, each of which has all the limitations previously discussed with respect to current disc drives.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an MO disc with an overcoat layer thick enough to enable handling of the disc without risk of data loss.

It is another object of the invention to provide an MO disc wherein data on the MO disc is accessed using a flying objective lens with a focus actuator, such that the thickness of the overcoat layer of the MO disc is not restricted by the necessity of focusing using the distance of the flying objective lens from the recording layer of the MO disc.

It is another object of the invention to provide an MO disc that operates in an MO drive that performs direct overwrite of data without initial erasure.

It is another object of the invention to provide an MO disc for use in a system that dynamically loads and unloads a magnetic head, so that no contact-start-stop events occur and the MO disc does not require lubrication.

It is another object of the invention to provide an MO disc for use in a system that dynamically loads and unloads a magnetic head so that no dedicated landing areas are required for the MO disc and the entire area of the MO disc may be used for data storage.

It is a further object of the invention to provide a two-sided MO disc wherein both sides of the MO disc may be accessed simultaneously and independently.

A magneto-optical (MO) disc and a method of accessing data stored on an MO disc is described. The MO disc comprises a substrate and a magneto-optical recording layer applied over the substrate. The MO disc further comprises an overcoat layer applied over the recording layer, having a thickness in a range between 25 nanometers and 200 microns.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a prior art magneto-optical (MO) data storage and retrieval system.

FIG. 2 is a diagram of a prior art MO data storage and retrieval system.

FIG. 3 is a diagram of a prior art MO data storage and retrieval system.

FIG. 4 is a diagram of an MO data storage and retrieval system and MO disc according to one embodiment of the present invention.

FIG. 5 is a diagram of a magnetic head suspension assembly according to one embodiment.

FIG. 6 is a top view of an MO drive according to one embodiment.

FIG. 7 is a partial side view of the MO drive of FIG. 6.

FIG. 8 is a partial end view of the MO drive of FIG. 6.

FIGS. 9A & 9B respectively show grooves formed on opposite recording layers of the same disk, wherein the grooves spiral in opposite directions.

FIG. 10 is a partial cross-sectional side view of a magneto-optical disc with an overcoat layer that comprises multiple layers in accordance with one embodiment of the present invention.

DETAILED DESCRIPTION

The present invention provides a magneto-optical (MO) disc drive and an MO disc. In one embodiment, recording is performed using a far field technique with a single objective lens focusing collimated laser light. The collimated laser light passes through the objective lens and then passes through an opening in a flying magnetic head that flies above the MO disc. Increased data storage densities are achieved over conventional storage techniques using protected MO discs. In one embodiment, the MO disc includes two opposing recording layers covered with overcoat layers. In this embodiment, both a flying magnetic head and an objective lens are situated on each side of the MO disc and independently actuated to allow simultaneous and completely independent access to both sides of the MO disc to perform overcoat incident recording wherein light is incident on an overcoat layer and focus is maintained with a focus actuator.

FIG. 4 is a diagram of an MO data storage and retrieval system 400 according to one embodiment of the present invention. In FIG. 4, elements that are similarly numbered except for an “a” or “b” are identical, or functionally equivalent. For example, objective lens 414 a is identical, or functionally equivalent to, objective lens 414 b.

Objective lens 414 a focuses collimated laser light beam 402 a. Flying height 406 a of magnetic recording head 418 a above disc 420 can be 0.05-5.0 microns, depending upon the specific embodiment. Magnetic recording heads 418 a and 418 b produce magnetic fields with relatively low inductance, allowing direct overwriting of data upon switching the magnetic field. Higher storage densities can be accomplished by modulating magnetic fields produced by magnetic recording heads 418 a and 418 b during write and read processes.

Magnetic recording head 418 a includes light channel 416 a which is an opening through the center of magnetic recording head 418 a. Slider 404 a is integrated with magnetic recording head 418 a and enables magnetic recording head 418 a to fly above MO disc 420 on an air bearing when disc 420 is spinning. System 400 records data on MO disc 420 using a known far field technique. In far field techniques, the focal distance (the distance from an objective lens to a focal point on a recording layer) is relatively large, and thus the objective lens is not in contact with, or in close proximity to, the recording layer. Also, the objective lens may be decoupled from the slider.

MO disc 420, in this embodiment, has spiral grooves in both recording layers. The spiral grooves on opposite recording layers spiral in opposite directions so that the spinning disc can be accessed simultaneously from both sides. (See FIGS. 9A & 9B.) Other embodiments use a two-sided MO disc with concentric grooves. Spiral groves are preferable when data to be stored and accessed is of a sequential nature. Concentric grooves are preferable when data to be stored and retrieved is of a less sequential and more random nature.

MO disc 420 includes a central substrate 412. On either side of substrate 412 are recording layers 410 a and 410 b, respectively. In one embodiment, recording layers 410 are made of a rare-earth transition-metal alloy, such as Tb—Fe—Co. Overcoat layer 408 a forms one surface of MO disc 420 and covers recording layer 410 a. Overcoat layer 408 b forms the opposite surface of MO disc 420 and covers recording layer 410 b.

In one embodiment, MO disc 420 includes additional layers. For example, in one embodiment, each recording layer 410 comprises multiple layers. Multiple-layer recording layers are known in the art and will not be described in detail. In one embodiment, MO disc 420 includes a reflective layer and a dielectric layer as is known in the prior art.

Overcoat layers 408 may be from approximately 25 nanometers to 200 microns in thickness. The thicker overcoat layer 408 is, the more the adverse effect of contamination on the surface of overcoat layer 408. Examples of contamination that typically impede the reading and writing process include dust and fingerprints. Greater depth of focus reduces the effect that particles on the surface of overcoat layer 408 have on focus. A consideration in choosing an overcoat 408 thickness is that the strength of the magnetic field produced by flying magnetic recording head 418 is reduced. Beyond a certain thickness, the size of the coil must be increased which causes an increase in the switching time of the magnetic field, which in turn reduces performance of the disc drive.

Overcoat layers 408 may be made with different materials depending upon design factors. For example, an overcoat material may be chosen to improve optical properties or to reduce the diameter of the light spot incident on the media (known as reducing spot size).

Overcoat layers 408, in one embodiment, comprise multiple layers. For example, in one embodiment, shown in FIG. 10 an overcoat layer 408 is made of a relatively thin layer 442 of silicon nitride as well as a relatively thick coating layer 441 of acrylic ultra violet (UV) resin. The silicon nitride layer is applied by a sputtering process and has a typical thickness of approximately 50 nanometers. The coating layer is applied by a spin coating process and has a typical thickness of approximately 10 microns. In another embodiment, overcoat layer 408 may be made of a thin layer of dielectric material (approximately 50 nanometers thick), such as silicon nitride, and a thicker layer of a transparent material such as polycarbonate applied by a bonding or laminating process.

In any case, the thickness of overcoats 408 may vary greatly and is not restricted to a very small dimension, as in the prior art where it is necessary to focus using the flying height.

Overcoat layers 408 do not require lubrication because with the apparatus and method of the present invention the possibility of contact between the media and the head is virtually eliminated. Therefore, in embodiments including a disc wiping mechanism, contaminants may be wiped from the surface of disc 420 before loading of the magnetic head.

Overcoats 408 may have an asperity height significantly greater than an asperity height of prior art discs used with an air-incident technique. Asperity height is a maximum height of a projection from a surface. For example, in one embodiment, the asperity height of overcoats 408 may range from zero to five microns. A much greater asperity height is possible for MO disc 420 because the flying height of recording heads 418 does not control focus, and therefore, the flying height can be relatively high. MO disc 420 thus has the advantage of being easier and less expensive to produce because the tolerances for surface finish are not very stringent.

In one embodiment, magnetic recording heads 418 are dynamically loaded so that no contact-start-stop events occur during operation. Also, the flying height of magnetic recording heads 418 is great enough so that accidental contact between magnetic recording heads 418 and MO disc 420 is extremely unlikely to occur. Therefore, no lubrication is required on the surfaces of MO disc 420. Another advantage is that the entire recording layers 410 of MO disc 420 are used to store data because no dedicated landing places are required for contact-start-stop events as in some prior art MO discs. Because prior art methods rely on contact-start-stop events, some area of the disc must be set aside for landing zones and cannot be used for storage. Higher overall data density is thus possible with MO disc 420 as compared to prior art MO discs.

Overcoat layers 408, in one embodiment, are applied by spin coating. In other embodiments, overcoat layers 408 may be applied by other methods, such as sputtering. Because overcoat layers 408 are thin relative to substrates of prior art discs such the disc of FIG. 1, overcoat layers 408 can be applied consistently so that birefringence does not occur on any portion of MO disc 420 when light passes through overcoat layers 408. Birefringence has a limiting effect on NA. Therefore, a benefit of eliminating birefringence is that NA can be increased, consequently increasing storage density on an MO disc.

Overcoat layers 408 are much thicker than those of discs used with air incident recording (as shown in prior art FIG. 3, for example). Therefore, MO disc 420 is much better protected from handling and environmental contamination than MO discs used for air incident recording.

Although overcoat layers 408 are thicker than those of discs used with air incident recording, overcoat layers 408 are much thinner than substrates of MO discs used with substrate incident recording. Therefore, system 400 has the advantage of having very little susceptibility to tilt as compared, for example to systems 100 or 200. Therefore, a higher NA lens may be used, making increased storage capacity and higher throughput possible.

In this embodiment, focus actuators controlling objective lens 414 a and magnetic head 418 a are completely independent from separate focus actuators controlling objective lens 414 b and magnetic head 418 b. Focus actuators are known in the art and are not shown for clarity. System 400 reads and writes respective sides of MO disc 420 with complete independence. For example, recording layer 410 a may be written to at the same time recording layer 410 b is read from by respective head assemblies.

In the embodiment of FIG. 4, MO data storage and retrieval system 400 includes two magnetic heads 418 and two lenses 414 for independently accessing respective sides of MO disc 420. In other embodiments, the MO data storage and retrieval system could include only one magnetic head 418 and one lens 420.

FIG. 5 is a diagram of slider/magnetic recording head suspension assembly 1000, according to one embodiment. A suspension assembly such as suspension assembly 1000 is used to suspend each slider 404 and magnetic head 418. Magnetic recording head suspension assemblies are not shown in FIG. 4 for clarity. Before read and write operations, slider/magnetic recording head suspension assembly 1000 is loaded by spinning the disc and then engaging, or loading, slider/magnetic recording head suspension assembly 1000. An air bearing is formed between the slider/magnetic recording head assembly 1010 and the spinning disc, and the slider surface never comes in contact with the disc surface. In other embodiments, the slider rests on a disc surface when the disc is not moving. In these embodiments, a slider rests on the disc surface until the disc reaches a certain revolutionary speed, after which an air bearing is formed and the slider is separated from the disc surface.

The slider also sweeps airborne particles on the surface of the MO disc that may otherwise interfere with or block light passing to the recording layer.

The magnetic circuit residing on the flying slider may operate effectively by flying several microns away from the recording layer and also from the media surface. Because the slider can fly several microns above the media surface, the media surface may be manufactured with less strict tolerances for surface asperity and flatness. For example, typical tolerances for surface asperity of the MO disc of the present invention are on the order of less than two microns.

During read and write operations, laser light beam 1008 passes through objective lens 1006 and through light channel 1013 in slider/magnetic recording head assembly 1010. In this embodiment, light channel 1013 is rectangular in this embodiment, but can be round or irregular in other embodiments. Light channel 1013 is surrounded by wire windings 1012. Slider/magnetic recording head assembly 1010 and windings 1012 are supported by gimbal 1004 and load beam 1002. Gimbal 1004 also has an opening 1005 that allows light to pass through gimbal 1004. The opening in gimbal 1004 may be rectangular, round, or irregular in shape.

FIG. 6 is a top view of disc drive 700 according to one embodiment of the invention. In this embodiment, Disc drive 700 includes two optical pickup/front-end electronics assemblies 716 a and 716 b. Assemblies 716, in this embodiment, are moved back and forth over respective sides of MO disc 710 by a linear focus actuator. Other embodiments could use other focus actuators, for example, rotary focus actuators.

Optical pickup/front-end electronics assembly 716 a is situated over one side of MO disc 710, and assembly 716 b is situated over the opposite side of MO disc 710. Each of the assemblies 716 are integrated optics assemblies. As is known, integrated optics assemblies include, in one unit, a focus actuator, a tracking actuator, a coarse actuator, optical components, and front end electronics. In the embodiment shown in FIG. 6, integrated optics are chosen, in part, for ease of assembly. Extreme precision is required to align the optical components of the mechanism. When integrated optics are used, alignment can be performed on an assembly, such as assembly 716 a, on a separate station before assembling the entire disc drive. This makes assembly faster and less expensive.

Other embodiments use split optics. Split optics include a moving portion and a stationary portion. The moving portion travels over the disc and includes an objective lens, a mirror, a fine actuator, a coarse actuator, and a focus actuator. The fixed portion includes a laser diode, a detector, optical components, and front end electronics.

In this embodiment, the focus actuator and the fine actuator are coupled to the coarse actuator. The coarse actuator performs relatively large movements laterally across the surface of the disc. The focus actuator moves axially with respect to the disc for focusing the laser light. The fine actuator performs small lateral movements, or microsteps, for keeping the focused laser light on a track of the disc.

In this embodiment, the magnetic head is coupled to the coarse actuator by compliant, flexible means. The magnetic head, in this embodiment, is decoupled from the focus actuator and from the fine actuator. The objective lens is coupled to the focus actuator and to the fine actuator. In alternate embodiments, the magnetic head is coupled to both the focus actuator and the fine actuator, both of which are, in turn, coupled to the coarse actuator.

Disc drive 700 includes carriage coil 702, return magnetic path assembly 704, and magnet 706. Spindle motor 708 engages MO disc 710 as explained more fully below. In this embodiment, dimension 718 is approximately 210 mm, dimension 714 is approximately 152 mm, and 712 is approximately 130 mm. Other embodiments of disc drive 700 could operate with MO discs of varying form factors. For example, disc drives embodying the invention could be used with MO discs as described herein, but with diameter form factors such as 80 mm, 90 mm, or 120 mm.

Disc drive 700 is an embodiment that includes two optical pickup/front end electronics assemblies. Other embodiments include only one optical pickup/front end electronics assembly that accesses one side of MO disc 710. These embodiments only read or write one side of a disc at one time.

FIG. 7 is a side view 800 of the disc drive of FIG. 6. Disc drive 700 is partially enclosed by top cover 802, bottom cover 804 and printed circuit board (PCB) assembly 808. Objective lens 810 of assembly 716 a is indicated. Assembly 716 a and 716 b are identical, or functionally equivalent. Magnetic head and suspension 818 are indicated for assembly 716 b. In this embodiment, the magnetic head is designed as a magnetic field modulation head. Magnetic field modulation techniques are known in the art. Coarse carriage coil 814 is indicated for assembly 716 b. Spindle motor 816 is shown engaged with disc 812. Reference number 806 designates the spindle motor in the disengaged position. Spindle motor 708 is disengaged during insertion or removal of disc 812. Spindle motor 708 moves up and engages with MO disc 710 after insertion of MO disc 710.

FIG. 8 is a diagram of end view 900 of disc drive 700. End view 900 references one magnetic head/suspension assembly 902, and objective lens 904. In this embodiment, magnetic head/suspension assembly 902 is mounted on the coarse actuator body and is not attached a focus actuator.

Top cover 906, bottom cover 912, and PCB assemblies 908 are also shown. Optical pickup/front end electronics assemblies 916 are shown on either side of MO disc 910. Carriage coil 914 for one assembly 916 is indicated. In this embodiment, dimension 924, the thickness of MO disc 910, is 0.6-2.4 mm. In this embodiment, dimension 918 is 5 mm, dimension 922 is 10 mm, and dimension 920 is 41.3 mm.

The embodiments shown perform parallel processing of data or redundant processing of data in one disc drive. Optical pickup/front end electronics assemblies 716 of FIG. 6 are operated by independent actuators, and the incoming data stream is divided between the two assemblies to perform independent reading and/or writing to either side of MO disc 710. A user can thus choose to increase throughput, or lower access time, by using parallel access. A user can alternately choose to access both sides of MO disc 710 redundantly in order to produce backup copies of data. When parallel access mode is chosen, client devices that perform command queuing can be serviced more quickly because commands in the queue can be smoothly executed even if they are not of the same type. For example, a write operation can be performed on one side of the MO disc 710 at the same time a read operation is performed on the opposite side of MO disc 710.

The embodiments shown perform MO recording using a far field technique with increased data density, simultaneous, independent access to two data storage surfaces, and direct overwrite capability. Other embodiments include only one optical pickup/front end electronics assembly and therefore do not perform simultaneous, independent access to two data storage surfaces.

The invention has been described in terms of particular embodiments. For example, the embodiments shown include an MO disc of a particular form factor and a disc drive with integrated optics and linear actuators. One skilled in the art, however, may make modifications and alterations to the specific embodiments shown without departing from the spirit and scope of the invention as set forth in the following claims. 

What is claimed is:
 1. A magneto-optical disc for data storage in an air-incident recording system, comprising: a substrate; a magneto-optical recording layer disposed over the substrate; and an overcoat layer having a thickness greater than or equal to 25 nanometers, but less than 100 microns, disposed over the magneto-optical recording layer; wherein during read/write operations of the air-incident recording system, light passes through the overcoat layer and is focused in a far field mode of operation onto the magneto-optical recording layer.
 2. The magneto-optical disc of claim 1, wherein the overcoat layer has an outer surface characterized by an absence of lubrication.
 3. The magneto-optical disc of claim 1, wherein the overcoat layer comprises multiple layers.
 4. The magneto-optical disc of claim 3, wherein the multiple layers include a first layer of a dielectric material and a second layer of a transparent material.
 5. The magneto-optical disc of claim 4, wherein the first layer comprises silicon nitride and the second layer comprises an acrylic ultra-violet resin.
 6. The magneto-optical disc of claim 4, wherein the first layer has a thickness of approximately 50 nanometers.
 7. The magneto-optical disc of claim 4, wherein the second layer has a thickness of approximately 10 microns.
 8. The magneto-optical disc of claim 4, wherein the transparent material comprises a polycarbonate.
 9. The magneto-optical disc of claim 7, wherein the transparent material comprises a polycarbonate.
 10. The magneto-optical disc of claim 1, wherein the magneto-optical disc is characterized by an absence of dedicated landing places.
 11. A magneto-optical disc for double-sided data storage in an air-incident recording system, comprising: a central substrate; first and second magneto-optical recording layers respectively disposed over opposite sides of the central substrate, the first and second magneto-optical recording layers each having a spiral or concentric groove to allow data access from both sides of the magneto-optical disc simultaneously and independently via independently actuated read/write head mechanisms; first and second overcoat layers respectively disposed over the first and second magneto-optical recording layers; wherein during read/write operations of the air-incident recording system, light emanating from a first side of the disc nearest to the first overcoat layer passes through the first overcoat layer and is focused in a far field mode of operation onto the first magneto-optical recording layer, and light emanating from a second side of the disc nearest to the second overcoat layer passes through the second overcoat layer and is focused in the far field mode of operation onto the second magneto-optical recording layer.
 12. The magneto-optical disc of claim 11, wherein the first and second overcoat layers each having a thickness in a range between 25 nanometers and 200 microns.
 13. The magneto-optical disc of claim 11, wherein the first and second overcoat layers each comprise multiple layers.
 14. The magneto-optical disc of claim 11, wherein the first and second overcoat layers each comprise a first layer of a dielectric material and a second layer of a transparent material.
 15. The magneto-optical disc of claim 11, wherein the grooves comprise spiral grooves that spiral in opposite directions with respect to each another.
 16. The magneto-optical disc of claim 14, wherein the dielectric material comprises silicon nitride and the transparent material comprises an acrylic ultra-violet resin.
 17. A magneto-optical disc for double-sided data storage in an air-incident recording system, comprising: a central substrate; first and second magneto-optical recording layers respectively disposed over opposite sides of the central substrate, the first and second magneto-optical recording layers each have a spiral or concentric groove to allow data access from both sides of the magneto-optical disc simultaneously and independently via independently actuated read/write head mechanisms; first and second overcoat layers respectively disposed over the first and second magneto-optical recording layers, the first and second overcoat layers each comprising multiple layers having a combined thickness in a range between 25 nanometers and 200 microns; wherein during read/write operations of the air-incident recording system, light emanating from a first side of the disc nearest to the first overcoat layer passes through the first overcoat layer and is focused in a far field mode of operation onto the first magneto-optical recording layer, and light emanating from a second side of the disc nearest to the second overcoat layer passes through the second overcoat layer and is focused in the far field mode of operation onto the second magneto-optical recording layer.
 18. The magneto-optical disc of claim 17, wherein the first and second overcoat layers each comprise a first layer of a dielectric material and a second layer of a transparent material.
 19. The magneto-optical disc of claim 18, wherein the dielectric material comprises silicon nitride and the transparent material comprises an acrylic ultra-violet resin. 